Laser direct current auxiliary ionization of an axially excited flowing gas

ABSTRACT

A high efficiency electric discharge gas laser which is capable of high power pulses and high repetition rates is disclosed. A region of high electrical conductance is provided in a laser gas along the axis of the laser optical cavity by a low current auxiliary ionization discharge, and a high voltage, high current pulsed power source produces an electric plasma having a population inversion throughout the region. Fins are positioned in the cavity to confine the cross section of the plasma and to maintain the plasma around the cavity axis. In one configuration in which the gas flow is transverse to the optical axis, a magnetic field which is orthogonal to both the direction of gas flow and the optical axis is used to counteract the downstream bowing effect in the plasma caused by the gas flow. In a high pressure embodiment of the invention, multiple electric discharges are arranged in parallel along the axis of the laser to provide the necessary pumping of the laser gas.

EEC at tts tent Buczek et a1.

Set. 4, 1973 DIRECT CURRENT AUXILIARY IONIZATION OF AN AXIALLY EXCITEDFLOWING GAS LASER Inventors: Carl J. Buczek, Manchester; Peter vt e asmb F m ngfi mi abe J. Freiberg, South Windsor, Robert J. Wayne, EastHartford, all of .C nnha.

United Aircraft Corporation, East Hartford, Conn.

Filed: July 21, 1971 Appl. No.: 164,544

Assignee:

' U.S. C1. 331/945, 330/43 Int. Cl. Hols 3/09, 1-101s 3/10, H015 3/22Field of Search 331/945; 330/43 OTHER PUBLICATIONS Huchital et al., IEEEJ. of Quantum Electronics Vol.

3, No. 9, Sept. 1967, pp. 378-9 QC44712.

Boczek et al., Applied Physics Letters Vol. 16, No. 8, 15 April 1970,pp. 321-3 QClA457.

Primary Examiner-David Schonberg Assistant Examiner-R. J. WebsterAttorney-Anthony J. Criso [5 7] ABSTRACT A high efficiency electricdischarge gas laser which is capable of high power pulses and highrepetition rates is disclosed. A region of high electrical conductanceis provided in a laser gas along the axis of the laser optical cavity bya low current auxiliary ionization discharge, and a high voltage, highcurrent pulsed power source produces an electric plasma having apopulation inversion throughout the region. Fins are positioned in thecavity to confine the cross section of the plasma and to maintain theplasma around the cavity axis. In one configuration in which the gasflow is transverse to the optical axis, a magnetic field which isorthogonal to both the direction of gas flow and the optical axis isused to counteract the downstream bowing effect in the plasma caused bythe gas flow. In a high pressure embodiment of the invention, multipleelectric discharges are arranged in parallel along the axis of the laserto provide the necessary pumping of the laser gas.

4 Claims, 3 Drawing Figures 4/4/24? OV/PVT DIRECT (:URRENT AUXILIARYIONEZATHON OF AN AXIALLY EXCITED FLOWIING GAS LASER BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to gas lasersand more particularly to a high efficiency gas laser capable ofproducing output pulses of high power at a high repetition rate.

2. Description of the Prior Art Various forms of gas lasers have beendiscussed and provded workable during the past few years, and much ofthe present laser effort is concerned with improving the characteristicsand feasibility of known workable systems. For example, laser pulsescontaining a peak power above one megawatt have been produced in acarbon dioxide gas laser system; additional research is being conductedto extend the power and efficiency of such systems.

Generally speaking, as more power is transferred to a laser medium, morepower can theoretically be removed from the medium in the form of laserenergy. However, there are limitations to the amount of useful powertransferable to a lasing gas. For example, in a nonflowing or sealed gaslaser system, the amount of useful energy which can be accepted by thelasing gas is limited in part by the ability of the gas to rejectunusable energy usually in the form of heat. As is well known in theart, lasing can occur when a nonequilibrium condition in which thepopulation of upper energy levels is caused to exceed the population oflower energy levels is established in a laser material; that is anenergy level distribution commonly referred to as a populationinversion. As lasing occurs, the lower energy level population tends toincrease, the upper energy level population tends to decrease and thedegree of population inversion is reduced unless a dynamic equilibriumcondition can be established. A main concern in the pumping of anefficient laser is to maximize the population of the upper laser leveland to minimize the lower laser level.

One method of reducing population in a lower laser energy level of a gaslaser medium is to transfer energy from the molecules in the lower laserlevel and cause these molecules to assume a different and still lowerenergy level. The energy transfer is often accomplished by interactingthe molecules with the walls of the laser cavity containing the gas.This system has been used extensively, however, the rate of energytransfer to the walls is limited by diffusion phenomena and thereforethe laser output power is similarly limited. One technique for enhancingthis rate used an axial magnetic field to increase the wall collisionrate as is discussed in the copending application of Bullis et al, US.application Ser. No. 216,303 filed on Jan. 7, 1962, and held with thepresent application by a common assignee.

An alternate method of reducing the lower laser level population is toflow the laser gas through the optical cavity, commonly referred to as aconvection laser system, thereby physically removing the gas in a lowerlaser energy level. While this method is effective, it can interferewith the transfer of pumping energy into the lasing gas. For example, inan electric discharge laser, ionization of the laser gas is essential tothe transfer of electrical energy to the upper laser energy level of thegas. When the lasing gas is being flowed through the optical cavity,particularly at high flow rates, ions are swept out of the electricdischarge region and the transfer of electrical energy to the laser gasis inhibited.

SUMMARY OF THE INVENTION A principal object of the present invention isto increase the overall efficiency of a gas laser having a pulsedoutput. A further object of the present invention is to increase theamount of pumping energy coupled into a gas laser with an electricdischarge, in order to increase the peak power contained in the laseroutput pulses.

According to the present invention, a low current electric discharge iscontinuously applied to a gas along the optical axis of a laser in orderto provide auxiliary ionization to the laser gas, the auxiliaryionization providing a path of high electrical conductance along whichpulses of high voltage, high current pumping energy are applied tocreate a population inversion in the laser gas.

A main feature of the present invention is that a steady state, directcurrent, auxiliary ionization is provided along a path over which themain excitation energy is applied to the laser gas in a pulsed manner.In addition, the electric discharge which provides the auxiliaryionization is operated as a low voltage, low current discharge. Thelaser has a pulsed output and is capable of high pulse repetition rateshaving excellent pulse-to-pulse reproducibility; the peak powercontained in successive pulses is high and it is essentially constantfrom pulse to pulse. The invention is further characterized in that thevolume of laser gas that is subject to auxiliary ionization is limitedto the approximate limits of the laser optical cavity. Further, theelectrical pumping system for creating the population inversion in thehigh pressure embodiment is operated at relatively low dischargevoltages.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of preferred embodiments as illustrated in theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified schematicdiagram of a transverse flow gas laser in accordance with the presentinvention;

FIG. 2 is a simplified schematic diagram of the gas laser shown in FIG.l with the auxiliary ionization circuit and the pulser ionizationcircuit; and

FIG. 3 is a simplified schematic diagram of a high pressure gas laser inaccordance with the present invention in which a plurality of pairs ofelectrodes is used to electrically excite the laser gas.

DESCRIPTION OF A PREFERRED EMBODIMENT The present invention can bepracticed with a transverse flow electric discharge gas laser as isshown in FIG. 1. A pair of resonator mirros l0, l2 define the ends of anoptical cavity shown by general reference character 14 having acentrally located optical axis 16. A pair of hollow electrodescomprising an anode l8 and a cathode 20 is located symmetrically aboutthe optical axis; the cathode is connected by wires 22 to both a directcurrent auxiliary ionization powersource 24 and a primary ionizationpower source 26. Disposed about the optical axis 16 is a plurality offins 28-33, each fin having a corresponding hole 34-39 which is alsosymmetrically located about the axis 116. The fins are preferably ofthin dielectric material; they are positioned substantiallyperpendicular to the optical axis and each fin extends several holediameters to each side of the optical axis in the gas flow direction.

In the operation of the device shown in FIG. 1, a laser gas having aflow direction 40 is passed between the electrodes 18, 20 in a directionparallel to the fins 28-33. An electric plasma is created between theelectrodes 18, 20 in the optical cavity 14 by the application of anelectric current which is typically less than about 5 milliamperes andis provided by the direct current power source 24. The power source 24is operated as follows. Initially the voltage is set above the thresholdvalue required to initiate breakdown in the laser gas between theelectrodes 18, 20.0nce a discharge has been initiated, both the voltageand current are reduced to a value just sufficient to maintain thedischarge between the electrodes thereby producing the necessaryauxiliary ionization. When auxiliary ionization is present in theoptical cavity, the need and drawbacks of generating ions to increasethe conductivity of the laser gas sufficiently at the time that thepulsed pumping power is applied as is normally the case, are eliminated.Thus, the invention makes possible laser pulses which have excellentrepeatability characteristics, including even the first several initialpulses.

The holes 34-39 control the size and the path of the discharge producedalong the optic axis by the power source 24. The size of the dischargemust be matched to the laser mode size which in turn is dictated by thecharacteristics of the optical cavity 14; the correlations betweenoptical cavity parameters and laser mode size are known in the art andform no part of this invention. However, if the cross section of theholes 34-39 is smaller than the cross section of the particular mode inwhich the cavity wants to oscillate, laser action will be quenchedalthough the electric discharge action will persist. On the other hand,if the cross section of the holes 34-39 is larger than the laser modecross section, laser action can occur but the discharge cross sectionwill exceed the laser mode cross section and some of the electricpumping energy transferred to the gas will be wasted thereby reducingthe system efficiency. In the present invention, it is important tomatch the tin hole size to the laser mode size, the latter beingdetermined by the optical cavity characteristics.

At appropriate times, the primary ionization power source 26 provides apulsed source of ionization potential between the electrodes 18, 20causing a population inversion in the ionized gas then present in theoptical cavity 14, said pulsed ionization having sufficient power tocause a population inversion which allows laser oscillation between themirrors l0, 12. The mirror 12 is a partially transmitting mirror and apulsed output 42 of laser energy is produced.

Maintaining a continuous flow of laser gas in the direction 40 removesexcess heat from the laser cavity and avoids buildup of gas molecules inthe lower laser level, a condition which is detrimental to the lasingprocess. If the laser gas is flowed very rapidly, in excess of about l0meters per second, the electric discharge does not remain symmetricallypositioned about the optical axis 16 between the electrodes 18, 2trather, the discharge tends to be blown downstream between each of theadjacent fins 284:3. Any portion of the electrically excited laser gaswhich is outside of the optical cavity,

is unused in the laser process, a wasteful and undesired condition thatcan be counteracted by applying a magnetic field along the optical axis16 and transverse to the flow of laser gases. This technique is taughtin the copending application of Pinsley et al., Ser. No. 216,302 whichwas filed on Jan. 7, 1972 and is held with the present invention by acommon assignee. Although the embodiment of the present invention asshown in FIGS. 1 and 2 is a transverse flow arrangement, an axial flowconfiguration has also been operated successfully.

FIG. 2 is a schematic diagram of the auxiliary ionization power source24 and the primary ionization power source 26. The direct current fromthe auxiliary ionization source must apply a sufficient voltage to breakdown the laser gas between the electrodes 18, 20, however, arcing in thedischarge is detrimental and to be avoided. An electrical resistance 44serves to limit the direct current drawn from a source 45 to a magnitudewhich is just sufficient to maintain the discharge thereby providingsufficient ionization for pulsed operation between the electrodes. Inorder to improve the reliability of the auxiliary ionization source andto isolate it from the primary ionization source, diodes 46 and 48 areused. The primary ionization source draws current from a dc supply 50; aresistance 52 serves to limit the current to an acceptable level while acapacitor 54 is being charged. A diode 56 provides suitable isolationfor the energy discharge from the capacitor when it is triggered by athyratron 58 which receives a signal from a trigger impulse 60.

It is known in the art that as the gas pressure is increased in anelectric discharge laser, higher excitation voltages are required inorder to maintain a preferred ratio of electric field strength to gaspressure; as the pressure increases, the lifetime of the upper laserlevel of the laser gas decreases and therefore the rise time of theexcitation pulses must be increased accordingly. If the rise time of theexcitation pulse is too long, some of the excited or upper energy levelswill revert to the lower energy levels by mechanisms which compete withthe laser process and the desired inverted population cannot beestablished.

The ideal ratio of electric field gradient to laser gas pressure remainsunchanged even though the pressure at which the laser is operated isincreased. Thus, low voltage power supplies can be used in high pressureapplication by locating several discharges in series along the opticalaxis.

FIG. 3 is a schematic diagram of an electric circuit suitable forimplementation of the present invention at relatively high gas pressure,typically about 1 atmosphere. A series of individual low voltagedischarges is established between the electrodes 18a-h, 20a-h to excitethe laser along the optical axis 16. In order to insure uniformity ofpulsed discharges, each electrode pair is driven by its own fast risehigh voltage capacitor 62a-h; each electrode pair also has a resistor64a-h to isolate the pairs of electrodes from one another and insureuniform pulsing between the electrodes. The capacitors 62a-h are chargedto a voltage in excess of that required for breakdown of the laser gasbetween the electrode pairs and a small resistance-capacitance timeconstant allows for short rise time pulses. The discharge breakdownacross the individual electrode pairs is controlled by a thyratron66-which is capable of a high peak current and a high pulse repetitionrate; activation of the thyratron causes the energy stored in thecapacitor 62 to be discharged instantaneously across the electrodes 18,20. A direct current auxiliary ionization power source 24 maintains adischarge between the electrode pairs to provide a high conductance pathfor the pulsed discharge, and between each pair of electrodes is a fin28 having an aperture 34 which limits the size of this discharge to theapproximate dimensions of the aperture.

One of the transverse flow systems operated in accordance with thepresent invention with carbon dioxide gas was shown to have better thana three-fold increase in both the average output power and peak powerper pulse when the same system was operated with and without auxiliaryionization. in this system, an auxiliary ionization power source havinga 30 kilovolt direct current capacity was operated with a milliamperecurrent limiting electrical resistance in the circuit. A power sourcecapable of a 40 amperes, at 30 kilovolts pulsed output at a 0.5-5microsecond duration and a repetition rate determined by the maximumduty cycle of 2 X provided the primary ionization power. With aseparation distance between electrodes of 30 centimeters and the opticalcavity pressures up to 50 Torr, the gas flow velocity was varied overthe range zero to 50 meters per second. The maximum average laser outputpower was observed when the electric field gradient to gas pressureratio was approximately 20 volts per centimeter Torr.

Although the invention has been shown and described with respect topreferred embodiments thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein, without departing from the spirit and scopeof the invention.

Having thus described a typical embodiment of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In the method of operating a pulsed gas laser in which gas iselectrically exicted in the resonant optical cavity of the laser, saidcavity having a characteristic internal, self consistent,electromagnetic field laser mode distribution configuration and anoptical axis about which a pair of spaced apart hollow electrodes iscoaxially disposed, the steps of:

flowing a quantity of the gas between said electrodes in a directionwhich is transverse to the optical axis;

providing a continuous direct current electric discharge between saidelectrodes to ionize the gas therebetween and provide a region of highelectrical conductance in the cavity along said axis;

providing a pulsed electric current discharge in said region of highconductance between the electrodes to produce in the cavity an electricplasma having a population inversion whereby stimulated emission ofpulsed laser energy is produced;

positioning the plasma substantially parallel to the optical axis;

confining the cross-sectional area of the plasma to substantially thecross-sectional area of the mode 6 distribution; and applying a magneticfield transverse to both the optical axis and the direction of gas flow.2. A gas laser apparatus for providing a pulsed output 5 of laser energywith a gas working medium comprising: a resonant optical cavity havingan optical axis and a characteristic, internal, self consistent,electromagnetic field laser mode distribution and capable of providingan output of pulsed laser power from an electrically excited gas; a pairof hollow electrodes coaxially disposed about the optical axis; meansfor delivering the gas to and removing the gas from the cavity and forflowing the gas in a direction which is transverse to the optical axis;means for applying to said electrodes a continuous electric currentwhich is capable of producing a region of high electrical conductancealong said axis in the gas provided to said cavity; means for applying apulsed electric current to said electrodes to produce along said axis anelectric plasma having a population inversion which is capable oflasing; means location in the flowing gas for positioning the plasmawith respect to the optical axis and confining the cross section of theplasma to substantially match the cross section of said modedistribution; and means providing a magnetic field transverse to boththe optical axis and the direction of the gas flow. 3. The laseraccording to claim 2 wherein said means for positioning and confiningthe cross section of the plasma comprises a plurality of fins which arespaced apart from one another and positioned perpendicular to theoptical axis, between said electrodes, each fin having a hole thereinand extending at least several hole diameters on either side of the holein the flow direction, each hole being disposed symmetrically about saidaxis and having a cross section substantially identical with the crosssection of said mode distribution.

4. In a flowing gas laser apparatus which includes: a resonant opticalcavity having an optical axis and a characteristic, internal, selfconsistent, electromagnetic field, laser mode distribution; electrodescoaxially disposed about the optical axis; means for delivering gas toand removing the gas from the cavity; and means for establishing anelectric potential between the electrodes to produce an electric plasmaalong the axis, the improvement comprising: means for positioning andconfining the cross section of the plasma comprising a plurality of finswhich are spaced apart from one another and positioned perpendicular tothe optical axis, each fin having a hole therein and extending at leastseveral hole diameters on either side of the hole which is disposedsymmetrically about the axis and has a cross section substantiallyidentical with the cross section of the mode distribution.

asaaa ggggg v UNITED STATES PATENT ()FFKCE CERTIFICATE 0F CQRRECTIONPatent No. 3.757.251 Dated Segtember 4, 1973 CARL J. BUCZEK, PETERjP.CHENAUSKY, Inventofls) ROBERT FREIBERG and ROBERT WAYNE It ,is certifiedthat: erfo'x': appears in the above-identified patent and that saidLetters Patent are hereby corrected as shown below:

C01. 1, li'n e" 32 ""that'" should read -'-"this' C01. 1, line 55 7"1962" should. read"-- 1972 C01. 2, 1ine 5 8 Q "mirros" should-readmirrors .signedu ar id sea le'd this 18th day of December 1973.

(SEAL): Attest: r

EDWARD M. FLETCHER, JR RENE D, TEGTI LEYER AttestingflfficerActing-Commissioner of Patents 2; UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,757,251 Dated Segtember 4, 1973CARL J. BUCZEK, PETERP. CHENAUSKY, Inventofls) ROBERT J. FREIBERGand-ROBERT WAYNE It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Col. 1, 11m; 32 "that" should read --"thiS-- Col. 1, line 55 "1962"should read'" 1972 Col. 2, line 58 "mirros" should read mirrors Signedand sealed this 18th day of December 1973.

(SEAL)Y, I V

Attest:

EDWARD M. FLETGI'ER, J'R RENE D. TEGTMEYER Attesting Officer ,Acting;Commissioner of Patents

1. In the method of operating a pulsed gas laser in which gas is electrically exicted in the resonant optical cavity of the laser, said cavity having a characteristic internal, self consistent, electromagnetic field laser mode distribution configuration and an optical axis about which a pair of spaced apart hollow electrodes is coaxially disposed, the steps of: flowing a quantity of the gas between said electrodes in a direction which is transverse to the optical axis; providing a continuous direct current electric discharge between said electrodes to ionize the gas therebetween and provide a region of high electrical conductance in the cavity along said axis; providing a pulsed electric current discharge in said region of high conductance between the electrodes to produce in the cavity an electric plasma having a population inversion whereby stimulated emission of pulsed laser energy is produced; positioning the plasma substantially parallel to the optical axis; confining the cross-sectional area of the plasma to substantially the cross-sectional area of the mode distribution; and applying a magnetic field transverse to both the optical axis and the direction of gas flow.
 2. A gas laser apparatus for providing a pulsed output of laser energy with a gas working medium comprising: a resonant optical cavity having an optical axis and a characteristic, internal, self consistent, electromagnetic field laser mode distribution and capable of providing an output of pulsed laser power from an electrically excited gas; a pair of hollow electrodes coaxially disposed about the optical axis; means for delivering the gas to and removing the gas from the cavity and for flowing the gas in a direction which is transverse to the optical axis; means for applying to said electrodes a continuous electric current which is capable of producing a region of high electrical conductance along said axis in the gas provided to said cavity; means for applying a pulsed electric current to said electrodes to produce along said axis an electric plasma having a population inversion which is capable of lasing; means location in the flowing gas for positioning the plasma with respect to the optical axis and confining the cross section of the plasma to substantially match the cross section of said mode distribution; and means providing a magnetic field transverse to both the optical axis and the direction of the gas flow.
 3. The laser according to claim 2 wherein said means for positioning and confining the cross section of the plasma comprises a plurality of fins which are spaced apart from one another and positioned perpendicular to the optical axis, between said electrodes, each fin having a hole therein and extending at least several hole diameters on either side of the hole in the flow direction, each hole being disposed symmetrically about said axis and having a cross section substantially identical with the cross section of said mode distribution.
 4. In a flowing gas laser apparatus which includes: a resonant optical cavity having an optical axis and a characteristic, internal, self consistent, electromagnetic field, laser mode distribution; electrodes coaxially disposed about the optical axis; means for delivering gas to and removing the gas from the cavity; and means for establishing an electric potential between the electrodes to produce an electric plasma along the axis, the improvement comprising: means for positioning and confining the cross section of the plasma comprising a plurality of fins which are spaced apart from one another and positioned perpendicular to the optical axis, each fin having a hole therein and extending at least several hole diameters on either side of the hole which is disposed symmetrically about the axis and has a cross section substantially identical with the cross section of the mode distribution. 